It is known that the data carried by magnetic disks are contained inside concentric circular tracks. They take the form of a succession of small magnetic cells, known as elementary cells, distributed over the entire length of each track. The magnetization in two consecutive elementary magnetic cells is in opposite directions and generally has the same modulus.
The means which enables data to be either written on the magnetic disks or read therefrom, or which enables both of these functions to be realized, is known as a transducer.
The trend in present-day development of magnetic disks is to seek ways of attaining radial densities of several thousand tracks per centimeter (measured along the diameter of the disks), and linear densities equal to or greater than 10,000 changes in magnetization direction per centimeter (measured along the circumference of the tracks).
A preferred method of writing data that enables such recording densities to be attained is known as perpendicular writing. In this method, the magnetization in the elementary cells is located perpendicular to the magnetic recording film of the disk. In this method, the magnetic medium comprising the film is an anisotropic magnetic medium having one preferred direction of magnetization, known as the direction of easy magnetization, perpendicular to the recording film.
To obtain very high linear and radial density of data, integrated or thin-film magnetic transducers are preferably used, the magnetic circuit of which include a main writing pole, which is magnetically excited by a coil, and an auxiliary pole.
Various types of integrated transducers including a main writing pole are known.
When the auxiliary pole and the main pole of the magnetic circuit are located on the same side with respect to the recording carrier, the magnetic transducer is known as the shoe type. An integrated transducer of this type is described in French Pat. No. 2.428.886, invented by Jean-Pierre Lazzari and filed June 13, 1978 by the Compagnie Internationale pour l'Informatique CII Honeywell Bull under the title "Support d'informations magnetiques a enregistrement perpendiculaire" ["Magnetic Data Carrier for Perpendicular Recording"]. The carrier travels past the transducer perpendicular to the plane of the thin magnetic films forming the pole pieces. Preferably, if the dimension of the poles measured parallel to the direction of travel defines the thickness of the pole pieces, then the thickness of the auxiliary pole is much greater than the thickness of the main writing pole (generally by a factor of more than 5), so that the cross section of the auxiliary pole is much greater than that of the writing pole.
The main writing pole is made of soft magnetic material, preferably anisotropic. In that case, the axis of difficult magnetization is perpendicular to the recording carrier. Writing data on the carrier is therefore performed by causing the carrier to travel therepast at a constant speed and sending a variable current, characteristic of the data to be written, through the coil associated with the writing pole. The magnetic field thus produced modifies the magnetic equilibrium of the pole pieces. With respect to the main pole, thre is a concentration of magnetic flux; the profile of the field is a function of the permeability of the pole and of its very slight thickness. The axis of easy magnetization of the magnetic recording film is perpendicular to the surface of it. The magnetic field component that is perpendicular to this surface is sufficiently intense that it can cause the reversal of the magnetization in this direction. With respect to the auxiliary pole, in contrast, the magnetic field and its component perpendicular to the surface of the film are of much lesser intensity than the same component with respect to the main writing pole. (In an initial approximation, the intensity of the fields produced in the vicinity of the poles varies in an inverse ratio to their cross sections, the magnetic flux being conservative.) Thus the magnetic state of the film at the level of the auxiliary pole is not modified. Only the main pole thus takes part in the writing process.
When the main pole and the auxiliary pole are disposed on opposite sides of the recording carrier, this is known as a monopole type of transducer, as defined by Professor Iwasaki in the journal IEEE Transactions on Magnetics, Vol. MAG.13, No. 5, September 1977, pages 272-277. In this case, the magnetic flux created by the main writing pole traverses the recording carrier and crosses to the auxiliary pole disposed on the other side of this carrier.
There are various versions of monopole transducer which do not include the auxiliary return-flux pole. Such transducers are described, for example, in European patent applications Nos. 82.304048.0, filed on July 30, 1982 by Fujitsu Limited under the title "A Perpendicular Magnetic Recording and Reproducing Head", and 83.401480.5 filed on July 19, 1983 by the American company, Vertimag Systems Corporation, under the title "Read-write Head with a Planar Coil or Coils".
Transducers of the shoe type and the monopole type function analogously to one another, and there is a trend at present to call such transducers either main-pole transducers or single pole transducers or heads.
Although the monopole transducers, in writing, are well suited to high density recording of data, since the length of the elementary magnetic cells that are recorded is substantially less than or equal to the thickness of the main writing pole, the same is not true for reading, where the signal-to-noise ratio of such transducers is relatively low. This is due to three essential phenomena:
(1) the fact that the coil associated with the main writing pole is disposed in a plane that is perpendicular to the recording carrier and to the direction of travel of the data;
(2) the very high density of the data recorded on the carrier and read by the transducer; and
(3) the distance between the carrier and the end of the writing pole that faces it.
In fact it has been found that the greater the density of the data, the more the intensity of the magnetic field created by the elementary magnetic cells of the carrier in the vicinity of the data decreases rapidly when the distance with respect to the surface of the carrier increases.
This is due to the strong magnetic coupling between the neighboring magnetic cells which result in the presence of a strong gradient of the field in the vicinity of the surface of the carrier.
Under these conditions, it is noted that the magnetic flux .PHI.i entering the main pole includes a useful portion .PHI.u, produced by the data one seeks to read, and a non-useful portion (noise producer) .PHI.e. This latter is due to the flux produced by the cells located on the same track adjacent to the cell facing which the pole is located, to the flux produced by the cells of tracks adjacent to the cell where the data being read are located, and to the flux produced by parasitic magnetic fields existing especially outside the recording carrier.
Under these conditions, the following facts are also observed experimentally by computer simulation. When the main pole is in a situation for picking up maximum flux .PHI.i (for example, when facing an elementary cell), the useful magnetic flux flows through it over only a very small height (a distance of all points of the pole with respect to the end of the pole disposed facing the carrier, this end being in turn located 2 to 3 tenths of a micron from the support), while contrarily it is subjected to the non-useful flux .PHI.e over a much greater height. This is a not inconsiderable cause of noise: a main pole on the order of 10 microns in height, for instance, is traversed by the useful flux over a height of only 1 to 3 microns.
Thus it will be appreciated that a coil disposed perpendicular to the support will pick up the useful flux over only a very small portion of its height, but contrarily will pick up the non-useful flux, created by the magnetic cells adjacent to those producing the useful magnetic flux, over a large portion of its height. As a result, single pole magnetic transducers as they are presently known are of little use for reading.